1. Field of the Invention
The present invention relates in general to multi-layer wiring board structures and more particularly to a multi-layer wiring board having a strip line type signal transmission structure with a view to attaining high-frequency drive.
2. Disclosure Information
In a multi-layer wiring board which is provided with grounding conductor lines constituting a grounding conductor wiring or pattern and signal circuit lines constituting a signal circuit wiring or pattern at different insulation layers of a substrate, as a multi-layer wiring board having a strip line type signal transmission structure, an increasing number of grounding conductor patterns are formed not into a solid shape that has no openings or breaks but into a grid or lattice shape having openings at regular intervals, irrespective of the material of the insulation layers, i.e., whether the material of the insulation layers is ceramic or resin.
This is because the grid shape enables the gas produced at the time of firing of ceramic or curing of resin to be vented more easily than the solid shape and also because the grid shape enables the insulation layers to be thinner and therefore the wiring board to be thinner in its entirety than the solid shape if the wiring boards are the same in the designed characteristic impedance and the insulation material.
However, when, for example as shown in FIGS. 1A and 1B the grounding conductor pattern is formed into a grid shape, the signal circuit lines S1 and S3 which are located just above and under the lines of the grounding conductor wirings G1 and G2 formed into a grid-shape differs in characteristic impedance from the signal circuit lines that are not so located, that is, the signal circuit line S2 parallel to the grounding conductor wirings G1 and G2 and located just above or under the openings Gop of the grid shape. This is because the condition of coupling of the signal circuit line S1 or S3 with the grounding conductor wirings G1 and G2 differs from the condition of coupling of the signal circuit line S2 with the grounding conductor wirings G1 and G2. More particularly, the characteristic impedance of a signal circuit line S2 in the case the lines of the grounding conductor wirings G1 and G2 are located just above and under the signal circuit line S2 differs from that in the case the openings Gop are located just above and under the signal circuit line S2. That is, the signal circuit line S2 has portions of a high characteristic impedance and a low characteristic impedance alternating in the longitudinal direction thereof. In the meantime, strictly speaking, the characteristic impedance of the signal circuit line S1 or S3 varies cyclically in the longitudinal direction thereof but its variation is small and negligible so the characteristic impedance of the signal circuit line S1 is herein considered as being constant.
Due to this, in case such a signal circuit line S2 is connected to another signal circuit line, reflection of a transmission signal occurs due to the mismatching of characteristic impedance at the joining portion. Further, also within the signal circuit line S2 there occurs reflection of a transmission signal due to variations of the impedance. Such a reflected signal becomes a noise for a transmitting signal, so that a correct information is not transmitted to a functional device such as an integrated circuit (IC), etc., thus causing an erroneous operation.
Accordingly, it is required by signal transmission in a mutli-layer wiring board that the characteristic impedance be as constant as possible. That is, in a multi-layer wiring board having a plurality of signal circuit lines, it is required that the difference of the maximum and minimum values of the characteristic impedance of the signal circuit lines (hereinafter referred to as maximum deviation value) including both the difference of impedance between different signal circuit lines and the variations of impedance of one signal circuit line in the longitudinal direction thereof, is within a predetermined range. In many cases, the maximum deviation value is expressed by rate, and a value resulting from the division of the maximum deviation value by a design value is used to express a maximum deviation from the design value, which maximum deviation is required to be within 10% or less.
In order to meet with the requirement, it is considered to design the circuit so that the signal circuit lines, as the signal circuit lines S1 and S3 shown in FIG. 1, run only just above and below the lines of the ground conductor wirings G1 and G2. However, in such a case, there is caused such a design restriction that the interval between the adjacent two signal circuit lines is restricted to the pitch of the grid of the grounding conductor pattern.
To solve the above problem, such an arrangement as shown in FIG. 4 has also been proposed in which the grounding conductor lines constituting a grid-shaped grounding conductor pattern G and the signal circuit lines constituting a signal circuit pattern S are placed one above another in such a manner as to form an angle with each other, i.e., in such a manner as to cross each other as disclosed in JP-A-4-127598 (Japanese Patent Publication before examination). But, this leads to a design restriction that the both patterns need to be arranged so as to form an angle with each other or cross each other.